Direct Acting Vasodilators
The one drug in this group, hydralazine, does not fit neatly into the other mechanistic classes, in part, because its mechanism of action is not entirely clear and it appears to have multiple, direct effects on the vascular smooth muscle. First, hydralazine causes smooth muscle hyperpolarization quite likely through the opening of K+-channels. It also may inhibit IP3-induced release of calcium from the smooth muscle sarcoplasmic reticulum. This calcium combines with calmodulin to activate myosin light chain kinase, which induces contraction. Finally, hydralazine stimulates the formation of nitric oxide by the vascular endothelium, leading to cGMP-mediated vasodilation.
Hydralazine, which is highly specific for arterial vessels, reduces systemic vascular resistance and arterial pressure. Indirect cardiac stimulation (e.g., tachycardia) occurs with hydralazine administration because of activation of the baroreceptor reflex.
Specific Drugs and Therapeutic Indications
The direct acting vasodilator that is used clinically is hydralazine. This drug is used in the treatment of hypertension and heart failure.
Hydralazine is used occasionally (although rarely alone) in the treatment of arterial hypertension. It is not first-line therapy for arterial hypertension. Its relatively short half-life, which necessitates frequent dosing, and its precipitation of reflex tachycardia make it undesirable for treating chronic hypertension. However, it is used in treating acute hypertensive emergencies, secondary hypertension caused by preecclampsia, and pulmonary hypertension. It is often used in conjunction with a beta-blocker and diuretic to attenuate the baroreceptor-mediated reflex tachycardia and renal sodium retention, respectively.
Hydralazine has a role in the management of heart failure because of its ability to reduce afterload and thereby enhance stroke volume and ejection fraction. When used in heart failure, it is given along with a diuretic and often with a nitrodilator.
Side Effects and Contraindications
Common side effects of hydralazine include headaches, flushing and tachycardia. Some patients (~10%) experience a lupus-like syndrome. Reflex cardiac stimulation can precipitate angina in patients with coronary artery disease.